Chlorinated poly(vinyl chloride) resin (hereafter "CPVC") has gained in popularity for use in extruded pipe, cable jacketing and structural components for buildings, mainly because of its stability. However, such popularity is tempered by the fact that commercially available CPVC having a chlorine (C1) content above 69% by weight (% by wt, or "% Ci"), referred to as "high-chlorine-concentration" (hereafter "high-Cl-conc" for brevity) CPVC, is as uneconomical to produce as it is difficult to process. Such high-Cl-conc CPVC is not prepared commercially. If made at all, on special order, it is made by the chlorination of an aqueous suspension of microporous PVC macrogranules ("granules" for brevity) in the presence of UV (ultraviolet) light radiation (referred to as the "photochlorination", "photo-slurry" or "water-slurry" process) over a much longer period of time than that required to make "standard CPVC".
The term "high-Cl-conc" CPVC is generally used in the art to refer to any CPVC having a C1 concentration greater than about 67%, and the term will be used herein in this general sense, as well as in the more specific sense it is used to refer to special order (at this time) CPVC having a Cl conc of at least 70%.
In some processes the UV-light may be varied during the process (referred to as "light ramping"); and in others, the temperature may be ramped ("temperature ramping"). In each case, the result is a macrogranular, microporous CPVC product characterized by a high concentration ("conc") of Cl per unit of surface area ("Cl/unit area"), in chains near the surface of a macrogranule. It has now been found that such "high-Cl-conc" CPVC is difficult to extrude or mold because of the relatively poor fusion characteristics of a mass of such granules, each of which granules has a higher conc of Cl near the surface than near its center.
Stated differently, the fusion temperature of granules of the same size and Cl content, of CPVCs made by the photo-slurry process, and, a two-step process which does not use light ("photoless") but a peroxide catalyst, respectively, is higher for the former because the C1/unit area of surface of the latter ("two-step" granules) is lower. All reference to "Cl content" herein refers to Cl chemically bound in the polymer chains of the resin. Because poor fusion is the "fingerprint" of non-uniform distribution, particularly exaggerated in a high-Cl-conc CPVC resin, granules of high-Cl-conc resin which are easily fused are deemed to have relatively uniform distribution of Cl in them, and are referred to as "being essentially uniformly chlorinated". To achieve such essentially uniform chlorination in a high-Cl-conc resin, one must overcome the "blocking effect" of the high-Cl-conc zone near the surface, which zone is produced by the water-slurry process.
Numerous processes, other than photochlorination, have been proposed, but the predominant commercial photo-slurry process is disclosed in U.S. Pat. No. 4,412,898 to Olson et al. Except for the liquid chlorination process as taught by Parker in U.S. Pat. No. 4,350,798, or, the solution chlorination process in which a solution of PVC in a solvent such as tetrachloroethane is photochlorinated, we are unaware of any process, other than the two-step process of the parent application, which (process) does not produce CPVC having a higher Cl conc near the surface, than near the center of a granule (referred to as "non-uniform" chlorination).
We know of no CPVC other than polymer "A" and two-step CPVC which has an overall concentration of Cl=72% by wt, yet is fusible, so as to provide a continuous phase at 170.degree. l C.
Such high-Cl-conc resin having a Cl content of at least 70% by wt, has been produced by us in a `photoless` two-step process. PVC homopolymer is chlorinated in an aqueous slurry, to yield a CPVC macrogranular product with a Cl content of at least 70% by wt, and a lower concentration of chlorine near the surface of a granule, than can be obtained with any commercial process. Most of all, such a CPVC resin is made without sacrificing product quality.
These conclusions, that the concentration of Cl in "two-step CPVC" is lower near the surface than the conc of Cl in any other commercially made CPVC (and CPVC made by the Olsen et al process in particular), are derived from the observation that granules of the former, having the same Cl concentration and size distribution, fuse to provide excellent parts at a lower temperature than those made with the latter, without losing any strength and with improved heat distortion properties relative to a waterslurry CPVC having the same chlorine content.
CPVC produced by a solution chlorination process ("solution CPVC" made by a process exemplified in Example 6 below), or, by a process described in U.S. Pat. No. 4,386,189 to Ackerman et al, (hereafter "the '189 process") are not commercial materials. Though the disclosure of the '189 patent is limited to CPVC having a maximum of 69% Cl (not a high-Cl-conc resin having 70-75% Cl) applicants nevertheless used the process to produce a CPVC having &gt;70% Cl because they surmised that such a CPVC would be the most closely comparable to the two-step CPVC. For reasons given below, it is confirmed that this CPVC made by the '189 process, referred to herein as `polymer "A"`, or "Ackerman CPVC", is most closely comparable to two-step CPVC. These two CPVCs are therefore compared and distinguished in detail herebelow.
Polymer "A" made by Ackerman et al had a specific gravity (sp gr) of 1,575 indicating a Cl content (covalently bonded) of 67%. They avoided photochlorination and the use of a swelling agent, by using a high-pressure process in which they chlorinated PVC resin in aqueous 10-30% hydrochloric (HCl) acid in the presence of a sufficiently large excess of liquid chlorine to form a distinct liquid chlorine phase; and, they catalyzed the reaction with a solution of organic peroxy compounds. Additional details of their process are provided in an example hereafter, which example was duplicated from the '189 patent for the purpose of comparing that resin with one made by the instant two-step process. As will be seen from test data presented herewith, the '189 process produced the most closely comparable (with two-step CPVC) CPVC resin.
The significance of Ackerman et al carrying out the reaction in the presence of an excess of chlorine and a large amount of aqueous (say 10 -24% HCl) cannot be overlooked because the concentration of peroxide catalyst is typically very low in the range from 10.sup.-4 to 10.sup.-5 mole/liter, and is affected by the concentration of hypochlorous acid (HOCl) in the slurry being chlorinated. When the slurry contains 10.9% HCl (the formality of electrolyte is 3.987) the concentration of HOCl is 1.1.times.10.sup.-6 mole/liter; when the slurry contains 18.8% HCl the concentration of HOCl is only 6.2.times.10.sup.-7 mole/liter (see "The Solubility of Chlorine in Aqueous Solutions of Chlorides and Free Energy of Trichloride Ion" by M. S. Sherril and E. F. Izard, Research Laboratory of Physical Chemistry, Massachusetts Institute of Technology, J. Am. Chem. Soc., Vol 53, pg 1667, May 1931).
Without the addition of HCl to the two-step system, we have less than 0.5% HCl, typically from 0.05 to 0.1 moles/liter of HCl (about 0.18% to 0.36% HCl) equivalent to 0.05 to 0.1 moles/liter of HOCl. This difference in HOCl conc between the '189 process and that used to make two-step CPVC, is much more than just 10 times (one order of magnitude); the difference is more than three orders of magnitude (10.sup.3).
The novel CPVC is produced by a two-step process which has several advantages: (1) it does not require the use of added aqueous HCl; this obviates the necessity to recover, purify and recycle the HC1; (2) it does not require the use of a large excess of chlorine, which obviates the necessity to recover, purify and recycle the chlorine; and, (3) the reaction proceeds with a controlled rate of generation of heat which requires little, if any heat removal initially, then proceeds at elevated temperature in excess of 100.degree. C. in the second step. These features make producing the two-step CPVC a commercial reality.
Further, not only is our high-Cl-conc CPVC product of excellent quality, but its Cl conc in the range from 70-75%, is distributed throughout the granules providing substantially uniform chlorination, with the result that the concentration of Cl near the surface is lower than it would be in any known, commercial aqueous chlorination process.